Rain, Snow and Snowmelt changes in recent decades in western North America

Dan Cayan (1,2) Mike Dettinger (2,1) Iris Stewart (1) Noah Knowles (2) 1 SIO 2 USGS

Climate of wet days is crucial to Western water.

An advance to earlier spring runoff timing from western snowmelt-dominated watersheds is observed since 1950’s.

What caused it? (precip timing, earlier melt, more rain, PDO shift)?

Western US Annual Precipitation

mountain precipitation is crucial resource

Topography

In the West, many of the wettest days occur during WARM storms when temperatures are near rain/snow

threshhold at middle (1000-2500m) elevations

PPT (mm)

In the Sierra Nevada, ~20 precip days provide 2/3 of annual precip

These days play out during a short season when N Pac storms are active.

mm/day

mm/day

Sacramento Headwaters

San Joaquin Headwaters Snowmelt

Hydrological effect of warmer mid-elevation and cooler high-elevation zonesis seen in comparing snowmelt runoff timingbtw Sacramento (lower elev)and San Joaquin (higher elev)

Noah Knowles

If the Sierrawarms by +2C

Yosemite loses

about 20% ofsnow covered

area

Mike Dettinger

With climate warming, California would lose much of its late spring Snowpack

By the end of the century California could lose half of its late spring snow pack due to climate warming. This simulation by Noah Knowles is guided by temperature changes from PCM’s Business-as-usual coupled climate simulation.

Noah Knowles

Earlier spring flows last 2-3 decades

“Center Timing” of snowmelt watersheds have advanced by 1-5 weeks earlier across West

Iris Stewart

center time

Precipitation Timing Trends

Earlier streamflow in recent decadesHas occurred despitePrecip coming later

But in the West,Precip has come later!

Streamflow Timing Trends

Earlier streamflow

Later streamflow

Atmospheric Circulation has driven warmer spring temperature and ealier runoff

Warming Trend Spring Iemp1950-1997

Warm phase of PDO yields earlier snowmelt

But, streamflow timing correlations are even stronger with spring temperature anomalies

TI is 4mo mean temp anompreceding &during month of CT

Partial correlations:streamflow timing fluctuations are quite strongly affected by spring temperature (TI)Warm TI anomalies Produce earlier CT’s.

PDO has evidently had an influence, but notas much as TI

Winter and especiallySpring temperatures were warmer in the western United States during 1977-1998relative to 1950-1976

But this map reflectsdry days (ppt=0) only—thisSuggests that there wasEarlier snowmelt

Dec-Feb

Mar-May

Examining temperature anomalies more carefully: 1977-1998 minus 1950-1976 Tmin dry days only

Dry days Wet days

Dec-Feb

But, in addition to dry days wet days were warmer during 1977-1998

Time spent in Warm-Wet patterns

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rce

nt

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da

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rce

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Time spent in Cool-Wet patterns

Mike Dettinger

Changes in synoptic patterns were involved in the shift toward earlier spring flows:

a different mix of wet days

Peak temp changes occurred in March and April

Dec-Feb

Jan 1977-1998 minus 1950-1976 Tmin

wet days (>6mm precip)

Feb

Jan

Apr

Mar

May

Summary

Rain vs. snow is crucial to water issues in the West. In California’s Sierra Nevada, only 20-30 days deliver most of the year’s water.

Timing of spring runoff advanced 1-3 wks in decades after 1977 r.e. decades before Causes:

Winter and esp Spring temp Certainly dry days but also wet days were involved—this

suggests not only early snowmelt but more immediate rainfall runoff occurred.

PDO contributed, but other climate anomalies resulting in warmer winters & spring were evidently stronger influences.

Since 1998, PDO reversed but spring temp and spring flows did not reverted

Need more & better monitoring at mid-high elevations